Ben Wirz wrote:
>
> Hello Everyone,
>
>         I need to produce a variable PWM output based on a 8 bit hex number
> that corresponds to the percentage duty cycle of the PWM.  My basic idea was
> use the interupt on overflow feature of timer0.
>
>         So the waveform would look like this:
>
>                   --------------------
>         |         |                   |
>         |         |                   |
>         ----------                    ---------------
>
>           $FF-PWM      PWM
>
>         The pseudo code would look something like this:
>
> Interupt_Service_Routine
>
>         If the output was low last time
>                 {
>                 set the output high
>                 set timer0 to $FF - PWM
>                 //Set Timer to Over Flow PWM latter
>                 }
>         else
>                 {
>                 set the output low
>                 set timer0 to PWM
>                 //Set Timer to Over Flow $FF-PWM latter
>                 }
>
> End_Service_Routine
>
>         Ok, well that seems simple enough to implement to me but what I
> would really like to do is try this with one of the cheaper noninterupt
> based PICs.   The problem is that the PIC will also be doing serial comms so
> it can't be totally tied up with making the PWM signal although there isn't
> alot else going on.
>
>         I would appreciate comment and suggestions on both the interupt and
> noninterupt based methods.
>
> Thanks Guys,
>
> Ben
>
> Ben Wirz                For Microchip PIC Products including the Simm Stick
>                         development system and Easy PIC'n Book, as well
> Wirz Electronics        as Motor Control, Polaroid Sonar Units, and more
> ben@wirz.com            Hobbyist Robotic & Electronic Supplies, visit:
>                         http://www.wirz.com/


OK Ben,
        I have done a lot of work in PWM for controling R/C servos In my
requirement there is a 15-20 mSec refresh of a 1-2 mSec pulse.

        My code generates the Pulse output based on the clock speed and I use
the fact that I have a 5 mSec window to do other things.

        I calculated the fastest ans slowest time through the rest of my code
and ensured that it was less than 5 Msecs. this then alowed me to
calculate a constant time to get the 20Msec Duty cycle.

        If you are interested in the code i can send it to you. BTW I control 2
O/Ps for two servos but the timings will allow up to 7 Servos
        IE 7*2 Msecs = 14 Msecs giving me a fixed time of 1 Msec and a variable
time of 5 Msec to do my other stuff.

--

Cheers Peter ..........

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